The invention relates to a boring tool according to the preamble of claim 1, especially a pointed boring tool, with an exchangeable cutting insert which is secured against loosening.
From European Patent B 0625395 there is known a boring tool of the class in question which is provided with a holder with an axial clamping slit, in which there is inserted a plate-shaped boring cutter as the exchangeable cutting insert in such a way that it bears against the clamping-slit base face. The nonpositive fastening of the boring cutter to the holder is achieved via at least one threaded stud, which is provided with a head and which engages in a bore in at least one of the clamping-slit branches as well as in a bore in the boring cutter. The central axis of the bore in the clamping-slit branch is offset in longitudinal and transverse direction of the boring tool relative to the central axis of the bore in the boring cutter. Furthermore, at least one of two circumferential rims of the threaded stud and boring-cutter bore facing one another is chamfered. In order to permit exact centering of the boring cutter relative to the axis of rotation of the holder, there is inserted, in a bore extending from the clamping-slit base face into the holder, a portion of a centering pin, whose other portion engages in a centering slit in the side of the boring cutter bearing against the clamping-slit base face, in order to center the boring cutter relative to the axis of rotation of the holder. The nominal width of the centering slit is made larger than that of the portion of the centering pin engaging in the centering slit. The direction and size of the offset of the central axes of the bore in the boring cutter and the bore in the clamping-slit branch are also chosen so as to ensure that, in the clamped condition of the threaded stud, only one centering-slit side face bears against one side of the centering pin when the boring cutter is in centered position.
By means of the configuration of the boring tool described in the foregoing it is indeed possible to make the tolerance range of the position of the centering-slit side face bearing against the centering pin as small as desired, but it suffers from the disadvantage of the highly complex manufacturing operations which must be performed to produce the centering slit in the boring cutter in view of the arrangement relative to the bore for receiving the centering pin in the holder. Another apparent disadvantage of the boring tool described hereinabove is that extremely high precision is necessary in machining the clamping slit and making the bore in the clamping-slit base face or the centering slit in the boring cutter in view of the minimal possible play between the boring cutter and the clamping slit. Even with very small tolerances, it is conceivable that, although the boring cutter, by means of the threaded stud, will indeed fit into correct position in the clamping slit immediately after assembly, the boring cutter will not always be able to bear stably against the clamping-slit base face in the desired manner during operation of the boring tool. For example, unwanted gaps may develop between the clamping-slit base face and the side of the boring cutter bearing against the said clamping-slit base face, and these gaps may be further enlarged by the vibrations occurring during operation of the boring tool.
Furthermore, when the boring cutter is in a centered position as described in the foregoing discussion, a centering pin of a cylindrical shape or a square shape in cross section bears against one centering-slit side face of the boring cutter. Aside from the high mechanical stress and strain on the centering pin during clamping of the boring cutter in the clamping slit of the holder by operation of the threaded stud, the centering pin must withstand large dynamic loads during operation of the boring tool. The boring tool is usually exposed to strong vibrations and stresses during its operation, for example due to discontinuous cutting. Under the influence of compressive forces resulting from the stresses and vibrations, deformation in the form of rectangular flattening develops on the centering pin, especially if it has cylindrical geometry, as well as on the side face in question in the clamping slit of the boring cutter. Furthermore, strong bending deformations due to the large compressive forces can be produced in the cylindrical centering pin and also in the centering pin having square cross section, since only a relatively small cross-sectional area can be achieved in the transverse direction of the boring tool, or in other words in the direction of the groove, even in the case of a centering pin with square cross section. Such deformations permit offset, albeit only slight, of the boring cutter in the clamping slit of the holder. This is detrimental to the intended exact and play-free centering of the boring cutter relative to the axis of rotation of the holder. The consequences can include development of inaccurate concentric running of the boring cutter, especially of its point, and thus shortening of the service life of the boring cutter and of the boring tool as a whole.
Aside from the highly complex manufacturing operations, another disadvantage is considered to be equipment complexity, from the viewpoint of compactness of the boring tool and of exact and true positioning of the boring cutter by means of the centering pin in the clamping slit of the holder.
The object of the invention is therefore to improve the boring tool of the class in question described in the introduction by means of an exchangeable cutting insert which is secured against loosening, such that the cutting insert can be centered and fixed relative to the holder in a manner that is simple from the equipment viewpoint but is nevertheless stable, exact and true.
This object is achieved by the inventive boring tool according to the features of the present invention. Advantageous embodiments of this boring tool are also the subject matter of the present invention.
A boring tool of the class in question with an exchangeable cutting insert as described in the preamble of claim 1 is generally operated at high cutting speeds. To ensure that the vibrations resulting from the high cutting speeds as well as the dynamic stresses and strains caused thereby can be withstood, such a boring tool is usually provided with a relatively large web diameter. The inventive improvement of the boring tool of the class in question now comprises in particular the fact that the cutting insert, which incidentally is preferably made of carbide metal, is provided with an extension which runs in the direction of the longitudinal axis of the boring tool and which reaches into a recess extending from the groove base face in the direction of the longitudinal axis to the shank portion and is pressed against a second stop face, provided in the recess, for fixation of the cutting insert in the direction of the transverse axis. The fact that the web diameter of boring tools of the class in question is generally relatively large is therefore utilized advantageously in the inventive improvement in order to form the recess for receiving the axial extension of the cutting insert. Since adequate space is available by virtue of the relatively large cross section of the boring-tool web, the recess can be formed without substantially reducing the strength and thus the stability of the boring tool.
Centering and fixation of the cutting insert on the holder are therefore accomplished via two stop faces, wherein the first stop face is intended to position the cutting insert primarily in the direction of the longitudinal axis and the second stop face is intended to position the cutting insert primarily in the direction of the transverse axis. By appropriate orientation of the first and second stop faces, for example by forming them as flat faces, each of which lies in a plane that is not normal relative to the longitudinal axis, it is naturally also possible for the first and second stop faces to position the cutting insert both in the direction of the transverse axis and in the direction of the longitudinal axis.
Compared with the conventional boring tool described in the introduction, in which the relatively small-area centering pin performs the function of positioning in the direction of the transverse axis, in the inventive boring tool both the first and also the second stop faces can be formed as relatively large faces, and so relatively large-area bracing of the cutting insert against the holder is possible both in longitudinal direction and in transverse direction. Accordingly, the high static and dynamic loads on the centering pin and boring cutter encountered heretofore in connection with bracing the boring cutter against the centering pin in the direction of the transverse axis can be reduced considerably, whereby any offset, caused by flattening of the centering pin, of the cutting insert in the direction of the transverse axis during the clamping process or during operation of the boring tool can be effectively prevented as a result. The inventive boring tool therefore permits much better concentric running and thereby ensures high quality in the production of bored holes.
The recess which extends from the groove base face to the shank portion into the holder can be made, for example, by boring or milling, the groove itself being made by techniques such as milling. The groove base face therefore has an area which is reduced by the cross-sectional area taken up by the recess. If the groove base face now functions expediently as the first stop face as in the conventional boring tool described in the introduction, the complexity of the manufacturing operations in connection with precision machining of the first stop face is reduced by virtue of the smaller area of the groove base face. In addition, the probability that unwanted gaps will develop between the first stop face and the corresponding side of the cutting insert is considerably reduced. At this stage it must be pointed out that there can function as the first stop face not only the groove base face but also, for example, the recess base face, although the groove base face would be advantageous with regard to stable positioning of the cutting insert in the groove and also with regard to precision machining of the first stop face.
By virtue of the inventive improvement of the boring tool of the class in question, the number of components of the boring tool on the holder, the cutting insert and the clamping element is also reduced. A centering pin for centering the cutting insert relative to the holder is no longer necessary, and so not only the complexity of the manufacturing operations but also the complexity of the equipment is reduced.
Just as is the case for the groove, the second stop face for fixation of the cutting insert in the direction of the transverse axis of the boring tool can be made without problems by techniques such as milling. As regards the complexity of manufacturing operations and also as regards the most intimate possible surface contact with the cutting insert, the first and second stop faces are advantageously formed as plane faces, wherein the first stop face is preferably formed as a plane face normal to the longitudinal axis and, provided the central axis of the receiving bore in the cutting insert is offset relative to the central axis of the bore in the at least one groove branch not only in the direction of the longitudinal axis but also in the direction of the transverse axis, the second stop face is also formed as a plane face normal to the transverse axis. The inventive boring tool, however, is not limited merely to such a configuration of the first and second stop faces. It would also be conceivable to form the first stop face and/or the stop face as a plane face or faces, which is or are aligned in a manner that is not normal to the longitudinal axis or transverse axis. Furthermore, the first stop face and also the second stop face can be formed as curved faces, although thereby the complexity as regards precision machining of these stop faces may be much greater. As regards the most intimate possible surface contact, in which as few unwanted gaps as possible will develop, the side faces provided on the cutting insert or on the extension of the cutting insert in order to be brought into bearing relationship with the first and second stop faces are advantageously complementary to the stop faces provided on the holder.
Good results in the chip-removing machining of high-strength materials can be achieved in particular if the boring tool is provided with at least one flute, which tapers out in the region of one of the tool faces of the cutting insert and merges flush thereinto. If in this case the recess has a cross section whose size is such that the recess merges into the flute, so that a tool such as a face miller can be introduced into the recess in the direction of the transverse axis via the flute, the second stop face, which is provided in the recess and which preferably is formed as a plane face normal to the transverse axis as described hereinabove, can be produced without problems. Accordingly, this configuration of the inventive boring tool proves to be advantageous in that, in contrast to the conventional boring tool described in the introduction, not only the first stop face but also the second stop face as well as the corresponding side faces provided on the cutting insert either as external faces or at least as faces that are readily accessible from outside can be produced or machined without problems in the inventive boring tool. In this way, therefore, all stop faces of the holder and side faces of the cutting insert can be created as plane faces with high surface quality, thus resulting in extremely precise and stable centering of the cutting insert on the holder.
As regards the greatest possible stability of the boring tool on the whole, the clamping element, which preferably is formed as a rotationally symmetric member, advantageously passes not only through the receiving bore formed in the cutting insert but also through a bore extending through both groove branches, in such a manner that the cutting insert is clamped between the two groove branches when the boring tool is in assembled condition. In this case the configuration of the clamping element as a clamping bolt with a head portion, which is inserted in a correspondingly adapted portion of the bore in one of the groove branches, with a shank portion, which passes through the receiving bore in the cutting insert, and with a threaded portion, which engages in a threaded portion of the bore in the other groove branch, offers a configuration of the clamping element that can be manufactured inexpensively and precisely.
Depending on the respective requirements, for example, the offset of the central axis of the receiving bore in the cutting insert relative to the bore extending through both groove branches can be such that the central axis of the receiving bore in the cutting insert is off set, relative to the central axis of the bore extending through both groove branches, in a manner parallel to the direction of the longitudinal axis and to the direction of the transverse axis. It is also possible, however, for the central axis of the receiving bore in the cutting insert and the central axis of the bore extending through both groove branches to lie respectively in one of two parallel planes disposed spaced apart in the direction of the longitudinal axis, wherein the central axis of the receiving bore in the cutting insert, viewed in the direction of the longitudinal axis, intersects the central axis of the bore extending through both groove branches at a particular angle.
The relative arrangement and orientation of the two central axes relative to one another also depends on the geometry of the clamping element and of the receiving bore in the cutting insert. By appropriate geometry of the clamping element and receiving bore in the cutting insert, allowing for the relative arrangement and orientation of the two central axes, it is possible positively to influence the distribution of compressive force exerted on the cutting insert via the clamping element along the inside wall of the receiving bore, and thus to achieve almost torque-free pressing of the cutting insert against the second stop face provided in the recess. If the cutting insert is in this case clamped against the groove branch of the holder by operation of the clamping element, it therefore experiences a clamping force wherein the lines of action of the clamping-force components are directed substantially only toward the first and also the second stop face.
In order to achieve the best possible distribution of pressing force along the inside wall of the receiving bore in the cutting insert, it has proved advantageous when the inclined face is formed by a conical structure of at least portions of the receiving bore in the cutting insert and/or of the clamping bolt. In this case, upon operation of the clamping bolt, the conical face provided in the form of a first wedge face on the one component cooperates with the face provided in the form of a second wedge face on the other component in the manner of a wedge-face mechanism, by which a large force effect (wedge effect) perpendicular to the wedge faces is achieved with a relatively small force, which can be applied by operation of the clamping bolt, in the direction of the central axis of the bore extending through both groove branches (with relatively long travel of the clamping bolt), whereby the cutting insert is pressed (with relatively short travel) against the first and also the second stop face.
To construct such a wedge-face mechanism as was mentioned in the foregoing, it is sufficient in principle if only the receiving bore or the clamping bolt engaging in the receiving bore has a conical structure at least in portions thereof, so that the conical face provided on one of the components (in other words on the clamping bolt or on the receiving bore in the cutting insert) intersects, at an acute angle, known as the wedge angle, the mating face provided on the other component (in other words on the receiving bore in the cutting insert or on the clamping bolt), which mating face could also have cylindrical structure, for example.
Preferably, however, the receiving bore in the cutting insert as one of the wedge faces has a conical inner face and the shank portion of the clamping bolt engaging in the receiving bore as the other wedge face has a conical outer face. Similarly, however, it would also be possible for the receiving bore in the cutting insert as the one wedge face to have a cylindrical inner face and the shank portion of the clamping bolt engaging in the receiving bore as the other wedge face to have a conical outer face.
In order to achieve an optimal stress condition and to apply the clamping force free of torque on the cutting insert, it is further advantageous for the central axis of the bore extending through both groove branches to intersect the longitudinal axis of the boring tool.
For the case that the clamping element is configured as a clamping bolt, which passes through a bore extending through both groove branches, it has further proved advantageous from the viewpoint of making the cutting insert bear with its full surface against the holder for there to be formed, below the base of the recess, a clamping slit extending from the recess base face in longitudinal direction to the shank portion, which slit lies in a first plane, which is turned by a particular angle relative to the plane defined by the transverse axis and the longitudinal axis, and for the central axis of the bore extending through both groove branches to be oriented normal to this first plane. By this feature the side faces of the cutting insert can be made to bear with their full surface against the respective groove side faces, even if the cutting insert is not matched exactly to the groove profile, especially if the side faces of the cutting insert are not exactly plane-parallel relative to the groove side faces. Therewith there is advantageously eliminated any possible cause of vibration, which is almost unavoidable in the prior art, of the cutting insert in the groove during critical machining tasks.
In initial experiments with a prototype, particularly good effects were found when the clamping slit described hereinabove lies in a first plane which is turned by an angle of, for example, between 0xc2x0 and 30xc2x0, preferably 10xc2x0 relative to the plane defined by the transverse axis and the longitudinal axis. Furthermore, a good clamping effect can be achieved when the ratio of the diameter of the boring tool to the depth extent of the clamping slit from the groove base face in the direction of the shank portion ranges between 2.5 and 6. A ratio of approximately 4 has proved particularly advantageous in this connection.
As a surprising effect in this inventive improvement of a boring tool of the class in question it has been found that, by the clamping slit formed in the holder in addition to the groove and recess, there is achieved, contrary to expectation, instead of the weakening of the boring tool that might initially have been expected compared with a boring tool comprising solid material in the region below the recess base face, an increase of the stability of the boring tool due to the cooperation of the forces and torques achieved by the clamping slit which is clamped together via the clamping bolt, whereby it is additionally ensured that the cutting insert is nonpositively seated over its entire surface in the holder in a manner that is absolutely free of play. At the same time this improvement permits advantageously larger manufacturing tolerances during production of the groove and thus more economic manufacture of the cutting-insert seat, and this is also true analogously for the manufacture of the cutting insert.
In a further advantageous embodiment, the groove, viewed in cross section, has a trapezoidal profile wherein the inside faces of the groove branches are inclined inwardly toward the point of the boring tool at a particular angle of inclination relative to the plane defined by the longitudinal axis and the transverse axis. In this connection the angle of inclination can range between 0xc2x0 and 5xc2x0, although an angle of inclination of about 1.5xc2x0 is preferred. If the side faces of the cutting insert are simultaneously modified in an analogous manner so that they have the same angle of inclination, this embodiment can, provided the cutting insert can be introduced in the direction of the transverse axis into the groove via the flute mentioned hereinabove, for example, permit easier assembly of the boring tool and thus a reduction of the manufacturing costs, since the cutting insert, after it has been inserted, is secured against falling out in the clamped condition by the trapezoidal groove profile.
As already mentioned, boring tools of the class in question with exchangeable cutting inserts are generally operated at high cutting speeds. In order to be able to withstand the vibrations resulting from the high cutting speeds and the dynamic stresses and strains caused thereby, such boring tools usually have a relatively large web diameter, thus leading to a decrease of the volume of the flute. Although the smaller flute volume results in the disadvantage of poorer removal of chips from the borehole, it can be eliminated, for example, in that chip removal is assisted by means of a coolant and/or lubricant such as a cutting emulsion for cooling and/or lubricating the boring tool, supplied to the boring-tool point under high pressure. The coolant and/or lubricant is preferably supplied via a coolant channel which runs substantially in the direction of the longitudinal axis, emerges at the head portion of the holder and, in the region disposed in the vicinity of the head portion of the holder, splits preferably into two secondary channels, each of which emerges at one end face of the groove branches.
The inventive construction of the boring-point holder is suitable in principle for all common cutting steps and geometries of the boring-tool point. This is primarily due to the fact that the functional faces necessary for fastening the cutting insert are kept as simple as possible. With the inventive fastening geometry, the ratio of the diameter of the boring tool, or in other words the distance between the two end cutting edges, to the dimension of the cutting insert in the direction of the longitudinal axis of the boring tool can be considerably increased, preferably to a range of about 1.5. It has been found that the holding means of the boring-tool point can be used without modification for boring tools which permit bore depths of up to 10xc3x97d. The ratio of the diameter of the boring tool to the axial length of a guide portion of the cutting insert is preferably increased to the range around 3.5.